Minecraft MOTD Parser's HtmlGenerator vulnerable to XSS

Summary

The HtmlGenerator class is subject to potential cross-site scripting (XSS) attack through a parsed malformed Minecraft server MOTD.

Context

Minecraft server owners can set a so-called MOTD (Message of the Day) for their server that appears next to the server icon and below the server name on the multiplayer server list of a player's Minecraft client. The Minecraft server sends the MOTD in the description property of the Status Response packet. The jgniecki/MinecraftMotdParser PHP library is able to parse the value of the description property, which can be either a string or an array of text components. By utilizing the aforementioned HtmlGenerator class, it is also able to transform the value into an HTML string that can be used to visualize the MOTD on a web page.

Details

The HtmlGenerator iterates through objects of MotdItem that are contained in an object of MotdItemCollection to generate a HTML string. An attacker can make malicious inputs to the color and text properties of MotdItem to inject own HTML into a web page during web page generation. For example by sending a malicious MOTD from a Minecraft server under their control that was queried and passed to the HtmlGenerator.

This XSS vulnerability exists because the values of these properties are neither filtered nor escaped, as can be seen here:

• https://github.com/jgniecki/MinecraftMotdParser/blob/0412f68eeb91729a00444a8d6c00c45623884aa5/src/Generator/HtmlGenerator.php#L49
• https://github.com/jgniecki/MinecraftMotdParser/blob/0412f68eeb91729a00444a8d6c00c45623884aa5/src/Generator/HtmlGenerator.php#L80

Proof of Concept

JavaScript code can be injected into the HtmlGenerator by parsing either a string via TextParser or an array via ArrayParser. The following code examples demonstrate the vulnerability by triggering the alert dialog of the browser.

XSS via TextParser

<?php

use DevLancer\MinecraftMotdParser\Collection\MotdItemCollection;
use DevLancer\MinecraftMotdParser\Generator\HtmlGenerator;
use DevLancer\MinecraftMotdParser\Parser\TextParser;

$motdCollection = (new TextParser())->parse('<script>alert("XSS on page load")</script>', new MotdItemCollection());

echo (new HtmlGenerator())->generate($motdCollection);

XSS via ArrayParser

<?php

use DevLancer\MinecraftMotdParser\Collection\MotdItemCollection;
use DevLancer\MinecraftMotdParser\Generator\HtmlGenerator;
use DevLancer\MinecraftMotdParser\Parser\ArrayParser;

$motdCollection = (new ArrayParser())->parse([
    [
        'color' => '#" onmouseover="javascript:alert(\'XSS when mouse pointer enters the span element\')"',
        'text' => 'Hover me',
    ],
    [
        'color' => '#000000',
        'text' => '<script>alert("XSS on page load")</script>',
    ]
], new MotdItemCollection());

echo (new HtmlGenerator())->generate($motdCollection);

Impact

If the HtmlGenerator class of this library is used, this XSS vulnerability can potentially affect:

• Players visiting Minecraft server list websites (of which there are several dozen online, written in PHP) that display the MOTD.
• Users visiting Minecraft server status websites to query information about a Minecraft server.
• Server owners managing their Minecraft server via a web interface that displays the MOTD, where the attack could be carried out by a malicious Minecraft server plugin that modifies the MOTD without the server owner's consent.

It is not clear if and which platforms depend on this library.

Remediation

I suggest converting all HTML special characters in the values of the color and text properties to HTML entities. The display of the HTML entities will still be correct in the browser, but the XSS vulnerability will be eliminated as the values will no longer be interpreted as HTML by the browser.

This could be achieved by introducing a new private escape function in the HtmlGenerator class:

private function escape(string $text): string
{
    return htmlentities($text, ENT_QUOTES | ENT_HTML5, 'UTF-8');
}

This function should be called in the following two lines:

• https://github.com/jgniecki/MinecraftMotdParser/blob/0412f68eeb91729a00444a8d6c00c45623884aa5/src/Generator/HtmlGenerator.php#L49 Change to: $tags['span'][] = sprintf('color: %s;', $this->escape($motdItem->getColor()));
• https://github.com/jgniecki/MinecraftMotdParser/blob/0412f68eeb91729a00444a8d6c00c45623884aa5/src/Generator/HtmlGenerator.php#L80 Change to: $value = sprintf($value, $this->escape($motdItem->getText()));